*3.1. Review of Control of DER, Protection Systems, Requirements for the Connection to the Grid and Parallel Operation with EPS*

Distributed energy resources, as well as energy storage systems, should fulfill the requirements in relation to relevant power system parameters including:


The issues of the impact of distributed energy resources on power system parameters were described by Conseil International des Grands Réseaux Électriques (CIGRE) in Reference [51] and Verband der Elektrotechnik Elektronik Informationstechnik (VDE) in Reference [52]. The European Commission established the requirements for connecting power sources to the grid in Reference [53]. The Agency for the Cooperation of Energy Regulators (ACER), in Reference [54], made it mandatory for transmission system operator (TSO) and distribution system operator (DSO) to include the mentioned requirements in network codes (NC). The network codes set out the necessary minimum standards and requirements that need to be followed when connecting the DER. A prominent example of the network code is the European Network of Transmission System Operators for Electricity (ENTSO-E) network code [55]. In the case of distributed generation with a connection point below 110 kV, the ranges of permissible limits of power system parameters are defined separately for four types of power generating modules, depending on the maximum power capacity. For the Central Europe networks, the different types of maximum capacity threshold from which a power generating module is a categorized are:


For the particular types of power generating modules at the point of connection to the grid, the following are defined:


The mentioned types of power generating modules are subject to predefined compliance simulations. Additionally, the network operators define the scheme and settings that are necessary to protect the network, taking into account the profiles of the generation units. The network code [55] mentions the following aspects of protection schemes: " [55]


All the mentioned aspects have been listed in order to highlight the technical aspects which should be taken into consideration when VPP concepts are created. The presented paper considers a case study of a VPP concept which uses resources that are localized in the distribution network. Some general studies about standards related to small generators are presented in Reference [56]. In order to highlight the details of the control strategy applied for small generators in the following subsections, selected issues are described in detail, including active power reduction strategies and voltage profile requirements. The described elements are used in the further investigations, which concentrate on the identification of the maximum capacity of the network for the energy storage connection and assessment of the losses of active power capacity in a photovoltaic power plant regarding reactive power control strategy.
